1. Field of the Invention
The present invention relates to pressure sensors. More particularly, the present invention concerns pressure sensors made of ceramic materials.
2. Discussion of the Related Art
Two types of ceramic pressure sensors are conventionally known, an electrostatic capacitive type and a variable resistance type. In the first type of pressure sensor, an electrode is formed on each of a diaphragm and a substrate. Upon application of pressure, the diaphragm is deformed to cause a change in the electrostatic capacitance between the electrodes. The measured pressure is then determined based upon a correlation between pressure and electrostatic capacitance.
Specific examples of this type of pressure sensor are described in Japanese Patent Unexamined Publication Nos. Sho 59-214727 and Sho. 59-148843. As shown in FIG. 21, these sensors comprise a ceramic diaphragm A and a ceramic substrate B, with thin-film electrodes C and D being formed on the opposing faces of the ceramic diaphragm A and the ceramic substrate B. The periphery of the ceramic diaphragm A is bonded to the ceramic substrate B by a glass spacer E so that the ceramic diaphragm A and the ceramic substrate B are spaced apart by a specified distance.
The pressure sensor having the construction described above is used such that the pressure to be measured will act on the outer surface of the ceramic diaphragm A. When the pressure is exerted, the ceramic diaphragm A deforms and, as a result, the electrode C formed on the inner surface of the ceramic diaphragm A is displaced with respect to the electrode D formed on the ceramic substrate B. Thus, the capacitance between the electrodes C and D changes by an amount sufficient to detect and measure the change in pressure.
Examples of prior art electrostatic capacitive pressure sensors having similar constructions are described in Japanese Utility Model Unexamined Publication Nos. Sho. 58-80540, Sho. 59-42938 and Sho. 60-51444.
A pressure sensor of the variable resistance type includes a resistor formed on a diaphragm which has a resistance value which varies in response to the deformation of the diaphragm when subjected to pressure changes. The pressure is measured based on the change of resistance of the resistor.
To manufacture the conventional ceramic pressure sensors, separately shaped and sintered diaphragms and substrates are bonded by an adhesive material such as glass. However, due to a thermal expansion mismatch between the adhesive, and the diaphragm or substrate, the strength of the adhesive will decrease as a result of repeated pressure measurements, and thus, the pressure resistance of the sensor deteriorates.
The thermal expansion mismatch between the adhesive, and the diaphragm or substrate presents another problem when the pressure sensor is used in a relatively hot atmosphere. For example, the diaphragm may deform, and thus cause detection errors, even when no additional pressure is actually being exerted on the diaphragm.
With a view to solving these problems, it has been proposed in Japanese Patent Unexamined Publication No. Sho. 63-292032 and Japanese Patent Examined Publication No. Sho. 63-9174, that the diaphragm and the substrate be assembled with an insert between the diaphragm and the substrate, which is formed of a material sublimable in the heat produced during the sintering of the assembly in order to form a predetermined space between the diaphragm and the substrate.
The problem with this approach is that the insert material is fairly expensive. In addition, the insert material must be shaped in conformance with the dimensions of the space to be formed prior to sintering. Further, when the diaphragm and the substrate are assembled by sintering, heating is necessary for a relatively long time in order to completely sublime the insert. Because of these difficulties, the manufacturing process is both complicated and expensive.